Electrosynthesis of nanofibers and nano-composite films

ABSTRACT

A method for producing an array of oriented nanofibers that involves forming a solution that includes at least one electroactive species. An electrode substrate is brought into contact with the solution. A current density is applied to the electrode substrate that includes at least a first step of applying a first substantially constant current density for a first time period and a second step of applying a second substantially constant current density for a second time period. The first and second time periods are of sufficient duration to electrically deposit on the electrode substrate an array of oriented nanofibers produced from the electroactive species. Also disclosed are films that include arrays or networks of oriented nanofibers and a method for amperometrically detecting or measuring at least one analyte in a sample.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with United States Government support underContract DE-AC0676RL01830 awarded by the U.S. Department of Energy. TheUnites States Government has certain rights in the invention.

FIELD

The present disclosure relates to the synthesis of nanofibers and filmsmade from the nanofibers.

BACKGROUND OF THE DISCLOSURE

Many methods have been reported for preparing oriented nanostructures,but most of these methods cannot be applied to organic polymermaterials. Oriented carbon nanotubes are prepared through chemical vapordeposition (CVD). Large arrays of oriented carbon nanotubes were grownfrom catalyst particles immobilized on porous silica or glass substrates(Li et al., Science, 274, 1996; and Ren et al., Science, 282, 1150,1998). A gas phase reaction or similar high temperature reactions havebeen used to prepare oriented nanorods of ZnO (Huanh et al., Science,292, 1897, 2001), Si (Yu et al., Physica. E., 9, 305, 2001), and siliconcarbide/nitride (Chen et al., J. Phys. Chem. of Solids, 62, 1567, 2001).A solution based synthesis method has been developed to prepare orientednanorods of ZnO₂ using a hydrothermal process (Vayssieres et al., Phys.Chem. B, 105, 3350, 2001).

Another widely investigated approach to prepare oriented nanoscalematerials is through templated synthesis, in which an inertnonconductive substrate material with oriented nanoporosity is used asthe mold or template (Huczko, Appl. Phys. A, 70, 365, 2000). Thetemplate nanoporosity is filled with the desired material. Subsequently,the substrate is partially or completely removed leaving a residue ofthe desired material that replicates the nanostructure of the template.Some of the mostly widely used templates include filtration membranes(e.g., polycarbonate films) and anodic alumina membranes.

The templated method was also used to prepare oriented rods or tubes ofpolypyrrole and polyaniline (De Vito et al., Chem. Mater., 10, 1738,1998; and Marinakos et al., Chem. Mater., 10, 1214, 1998). Recently Gaoet al. (Angew. Chem. Int. Ed., 39, 3664, 2000), used oriented carbonnanotubes prepared by a CVD process as the template to electrochemicallydeposit a thin polyaniline polymer coating on the surface of the carbonnanotubes. This method produced a carbon nanotube/polyaniline compositewith reportedly good electrical conductivity and electrochemicalactivity.

Electrospinning has also been used for preparing conducting polymernanofibers (Doshi et al., J. Electros., 35, 151, 1999; and Reneker etal., J. Appl. Phys., 87, 4531, 2000). In electrospinning, a high voltageis applied to the tip of a syringe until a jet is produced. The chargedpolymers in the jet repel each other to form thin fibers.

Despite all of these efforts, a need continues to exist for synthesismethods for controlling the morphology of nanostructures, particularlythose made from conducting polymers. Ideally, a nanostructure synthesiswould be templateless and involve liquid phase processing that can beused as the reaction medium for a wide variety of materials.

One class of conducting material that has attracted increasing attentionare polynuclear transition metal hexacyanometallates by virtue of theirelectronic, electrochemical, and spectrochemical properties. Electrodesformed from films of hexacyanometallates have been made, but theirinstability and electrical properties remains a critical issue. Forexample, composite modified electrodes have been made with conductingpolymer films that include iron (III) hexacyanoferrate (also known as“Prussian blue”) as a dopant or inorganic conductor (Ogura et al., J.Electrochem. Soc., 142, 4026, 1995; Koncki et al., Anal. Chem., 70,2544, 1998; and Ikeda et al., J. Electroanal. Chem., 489, 46-54, 2000).Composite films made from polyaniline and iron (III) hexacyanoferrateare described in U.S. Pat. No. 5,282,955 (Leventis et al.). Leventis etal. does not describe a synthesis method for controlling the morphologyof composites to produce an oriented nanostructure and theelectrochemical deposition of the polyaniline is accomplished by quicklycycling (e.g., from 10-1000 millivolts/second) the electrode between twovoltages. Composite films made from poly(3,4-ethylenedioxythiophene) andiron (III) hexacyanoferrate are described in Noel et al., “Compositefilms of iron (III) hexacyanoferrate andpoly(3,4-ethylenedioxythiophene)”, Journal Electroanalytical Chemistry489, 46-54 (2000). Noel et al. does not describe a synthesis method forcontrolling the morphology of composites to produce an orientednanostructure and the electrochemical deposition of thepoly(3,4-ethylenedioxythiophene) is accomplished by quickly stepping thevoltage to increasingly higher voltages.

One application of iron (III) hexacyanoferrate-modified electrodes is inthe construction of biological and chemical sensors. More specifically,there is an increasing need for more sensitive and selective detectionor measurement of peroxide compounds in clinical, pharmaceutical, food,industrial, and environmental applications. For example, amperometricdetermination of hydrogen peroxide is of great importance, inspired bythe wide use of peroxide sensors in bioanalytical systems based onoxidase-type enzymes. In oxidase-catalyzed reactions, oxygen andhydrogen peroxide are the substrate and product, respectively. Hydrogenperoxide determination is also important to ensure the safety andquality of pharmaceutical and cosmetic formulations. In addition,monitoring of organic (hydro)peroxides formed during the reaction ofozone with organic compounds in the atmosphere and drinking water ordirectly released into the environment from numerous industrialprocesses is desirable because of their adverse health effects.

Amperometric determinations of peroxides are generally performed byoxidation at +0.6 to +0.7 V vs. Ag/AgCl on a platinum electrode (forH₂O₂) (Guilbault et al., Anal. Chim. Acta 64, 439-455, 1973) or byreduction at −0.3 to −1.0 V vs. Ag/AgCl on gold/mercury amalgam orglassy carbon electrode (for organic and lipid hydroperoxides) (Cosgroveet al., Analyst, 113, 1811-1815, 1988; Funk et al., Anal. Chem., 52,773-774, 1980). At such large overpotentials, substances present inbiological samples such as ascorbic acid, uric acid and acetaminopheninterfere under oxidation conditions, while oxygen, benzoquinone, andnitrobenzene interfere at such reduction potentials. Low selectivity,therefore, is a major limitation in amperometric determinations.

One approach for addressing this problem is to use selectiveelectrocatalysts that lower an overpotential of hydrogen peroxideelectrooxidation to an appropriate level that prevents the discharge ofother substances at the applied electrode potential. Iron (III)hexacyanoferrate has been identified as a possible selectiveelectrocatalyst. For example, Garjonyte et al., Sensors and Actuators B46, 236-241 (1998) describe a carbon paste electrode modified by ferroushexacyanoferrate that electrocatalyzed the cathodic reduction ofhydrogen peroxide. Karyakin et al., “Prussian Blue-BasedFirst-Generation Biosensor, A Sensitive Amperometric Electrode forGlucose”, Anal. Chem., 67, 2419-2423, 1995, describe a glucoseamperometric biosensor made by glucose oxidase immobilization onto aPrussian blue-modified electrode with a perfluorosulfonate ionomer(Nafion® membrane) layer. In the sensors described by Garjonyte et al.and Karyakin et al. the Prussian blue sensing sites are only accessibleby the analyte on a two-dimensional electrode surface and, thus,miniaturization of the sensor is difficult due to the limited totalsensing surface area.

SUMMARY OF THE DISCLOSURE

Disclosed herein are various electrosynthesis methods for controllingthe morphology of nanostructures. In particular, there are describedmethods for producing an array of oriented nanofibers. According to onevariant, a solution is formed that includes at least one electroactivespecies. An electrode substrate is brought into contact with thesolution. A current is applied to the electrode substrate that includesat least a first step of applying a first substantially constant currentdensity for a first time period and a second step of applying a secondsubstantially constant current density for a second time period. Thefirst and second time periods are of sufficient duration to electricallydeposit on the electrode substrate an array of oriented nanofibersproduced from the electroactive species.

In another variant, the electroactive species may be an organic monomer.The first substantially constant current density is applied for a firsttime period to deposit seed nuclei of an electroactive polymer on theelectrode substrate, wherein the electroactive polymer is produced fromthe organic monomer. The second substantially constant current densityis applied for a second time period to grow organic polymer nanofibersfrom the deposited seed nuclei.

A further approach involves depositing substantially spherical particleson an electrode substrate, forming a solution that includes at least oneelectroactive species, contacting the solution and the electrodesubstrate, and electrodepositing the electroactive species on theelectrode substrate to produce an array of nanofibers.

Also described is a method of producing a film on a substrate thatincludes applying to an electrode substrate an electrical current regimethat includes successive steps of successively reduced current amountsunder conditions sufficient to electrochemically deposit a film on theelectrode substrate.

A further method of forming a film on a substrate involveselectrodepositing a nanoporous array of oriented nanofibers on asubstrate. A second substance is electrodeposited within the nanoporesof the nanoporous array to form a film.

Nanofiber arrays and films are also described in addition to the methodsidentified above. For example, there is disclosed a substrate definingat least one surface having deposited thereon an array of freestandingoriented organic polymer nanofibers, wherein the array was produced byliquid phase processing and without a template. Also described is a filmcomprising a network of three-dimensionally oriented conducting polymernanofibers, wherein the individual nanofibers have a substantiallyuniform cylindrical shape and there is substantially no branching of theindividual nanofibers. The film may be a composite film wherein thenanofiber network defines nanometer-sized voids into which a secondsubstance is at least partially received.

The composite film may be used to modify an electrode to produce achemical or biological sensor. For example, there is described a methodfor amperometrically detecting or measuring at least one analyte in asample. An electrode is provided that is at least partially coated witha composite film that includes an oriented nanoporous conducting polymermatrix and a metal hexacyanometallate at least partially dispersedwithin the oriented nanoporous conducting polymer matrix. An electricalpotential is applied to the electrode and the sample is contacted withthe electrode under conditions sufficient to amperometrically detect theanalyte.

An additional application for the composite film is in the separation ofcesium ions from a mixture.

The disclosed methods, arrays, films, and devices will become moreapparent from the following detailed description of several embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments will be described in more detail with reference tothe drawings described below.

FIGS. 1A-1D are field scanning emission electron microscope (SEM)micrographs of oriented polyaniline nanofibers on a platinum substrate.FIG. 1A shows a perpendicular top view at 10,000× magnification; FIG. 1Bshows a perpendicular top view at 30,000× magnification; FIG. 1C shows atop view of a sample tilted at about 40° at 10,000× magnification; andFIG. 1D shows a top view of a sample tilted at about 40° at 30,000×magnification. A scale bar is included at the bottom right hand cornerof FIGS. 1A-1D.

FIG. 2A is a SEM micrograph of polyaniline particles deposited on aplatinum substrate after an initial deposition step.

FIG. 2B is a SEM micrograph of polyaniline electrochemically depositedon a platinum substrate without a stepped deposition process.

FIGS. 3A-3D are SEM micrographs of oriented polyaniline nanofibersdeposited on colloidal silica particles. FIG. 3A shows a top view of onesample area tilted at about 40° at 10,000× magnification; FIG. 3B showsa perpendicular top view at 30,000× magnification; FIG. 3C shows a topview of another sample area tilted at about 40° at 10,000×magnification; and FIG. 3D shows a top view of a sample tilted at about40° at 30,000× magnification.

FIG. 4 is a graphical depiction of cyclic voltammograms of differentpolyaniline film morphologies. Curve (a) represents oriented polyanilinenanofibers on a Pt substrate synthesized according to methods disclosedherein, (b) represents oriented polyaniline nanofibers on silica spheressynthesized according to methods disclosed herein, and (c) represents aconventional polyaniline film deposited without the step-wise currentprocess. The y-axis depicts the redox currents generated byoxidation/reduction of polyaniline. The x-axis depicts the voltageapplied to the working electrode.

FIG. 5 is a graph showing currents produced by a sensor electrode as aresult of the catalytic reduction of hydrogen peroxide at the surface ofthe sensor electrode. The sensor electrode was prepared by forming apolyaniline/iron (III) hexacyanoferrate composite film on the electrodesurface.

FIG. 6 is a graph showing the response of the sensor electrode used inFIG. 5 to six repetitive injections of 50 μL hydrogen peroxide solutionhaving a hydrogen peroxide concentration of 50 ppm.

FIG. 7 is a schematic diagram of an amperometric detection system thatutilizes a sensor electrode prepared by forming a polyaniline/iron (III)hexacyanoferrate composite film on the electrode surface.

FIG. 8 is a plan sectional view of a microelectrochemical cell used inthe system of FIG. 7.

DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS

For ease of understanding, the following terms used herein are describedbelow in more detail:

“Nanometer” or “nanometer-sized” denotes a material or construct whoselargest dimension is less than one micron.

“Oriented nanofibers” indicates that substantially all nanofibers in aspecific structure or array are arranged parallel to each other in alongitudinal direction (“unidirectionally oriented”) or in awell-defined three-dimensional network (“three-dimensionally oriented”).In other words, the nanofibers are not randomly spatially arranged withrespect to each other. In most instances, the nanofibers describedherein grow in a generally perpendicular direction relative to thesupporting substrate surface and there is very minimal, if any,branching of individual nanofiber strands.

“Solution” includes various heterogeneous mixtures such as suspensionsor dispersions as well as true homogeneous solutions.

In addition, although the term “nanofiber” is typically employed in thisdisclosure, “nanowire” is also appropriate nomenclature, particularlyfor the filament structures grown from relatively flat planar substratesurfaces.

The above definitions are provided solely to aid the reader, and shouldnot be construed to have a scope less than that understood by a personof ordinary skill in the art or as limiting the scope of the appendedclaims.

A stepwise electrochemical deposition process may be used to synthesizethe nanofiber structures described herein. Each step typically involvesapplying a substantially constant current (i.e., galvanostatic) with apredetermined current density to an electrode that is contacting anelectrolyte solution. The current applied in each step is heldsubstantially constant for a sufficient period of time to synthesize thedesired nanofiber structure.

The process may include at least two electrochemical steps. Although notbound by any theory, it is believed that the first step deposits on theelectrode substrate a relatively large number of seed nuclei of theelectroactive species present in the electrolyte solution. The step(s)subsequent to the first step grow the nanofiber from the nucleationsites created in the first step. If the electroactive species is anorganic monomer, then a polymer is formed at the seed nucleation sitesand the subsequent step(s) continue the growth on polymer nanofiber.

Utilizing a current density in the first step that is greater relativeto subsequent steps can provide improved nucleation. Furthermore,successive reduction of the applied current density in successive stepsenhances formation of uniform nanofibers. In particular, reducing thecurrent density in a stepwise manner prevents the formation of randomlyoriented nanofibers. For example, the current density applied in thestep subsequent to the first step may be about 20 to about 80% less thanthat applied in the first step. For each succeeding step, the currentdensity may be further reduced by about 20 to about 80% for each step.

According to certain synthesis schemes, the substantially constantcurrent density in the first step may be from about 0.06 to about 0.1mA/cm² for about 10 to about 40 minutes. The substantially constantcurrent density in the second step may be from about 20 to about 80%less than that applied in the second step for about 100 to 240 minutes.And the substantially constant current density in the third step (ifdesired) may be further reduced by about 20% to about 80% less than thatapplied in the second step for about 100 to about 240 minutes. If thefirst step is prolonged, a much higher density of nuclei may bedeposited on the surface but nanofibers may not form in the subsequentstep(s). Moreover, thick worm-like and highly branched fibers in theexcess of a few hundred nanometers begin to randomly form.

The electrochemical deposition may be performed at any temperature thatdoes not deleteriously interfere with the electrochemical processes. Forexample, the deposition may be performed from about 10° C. to about 60°C.

The substrate upon which the nanofibers are generated typically is aconducting or semiconductor material that can act as an electrode.Illustrative electrode materials include, for example, metals, carbon,conductive metal oxides, semiconductors, conductive plastic materials,and similar materials. Particularly useful electrode materials includean inert metal such as platinum, gold, silver, rhodium, palladium,ruthenium, titanium, or stainless steel; carbon (e.g., glass carbon); aconductive metal oxide such as tin oxide, indium oxide, cadmium oxide,or antimony oxide; a semiconductor (e.g., silicon or germanium); or aseparate contiguous body of a base metal or ceramic, glass or plasticmaterial which is coated on at least one surface with the foregoingmetal, carbon, conductive metal oxide, semiconductor or conductiveplastic materials.

At least one surfactant may be coated on the electrode substrate surfaceprior to electrodeposition to improve the wetting of the surface by theelectrolyte solution. The surfactant molecules may also generatenucleation sites for growing the nanofibers. Any surfactant that can wetmetal or similar electrode surfaces is suitable. A suitable surfactantis sodium dodecyl sulfate and similar surfactants.

Substrates with high conductivities are especially suitable. Lowerconductivity substrates such as silicon substrates may be modified toincrease their conductivity by known techniques such as, for example,doping or surface modification with a highly conductive material.Illustrative highly conductive materials include gold, platinum, or tinoxide.

The nanofiber orientation may be controlled by varying certain processparameters. For example, varying the substrate surface topography cancorrespondingly alter the nanofiber orientation.

In one instructive illustration, the electrode surface is initiallycoated with substantially spherical particles, particularlynanometer-sized particles, prior to electrochemical deposition of thenanofiber-forming substance. The substantially spherical particles maybe colloidal silica spheres deposited on the electrode surface. Othermaterials that could provide substantially spherical particles includealuminum oxide and titanium dioxide. The coated electrode surface thenis contacted with the solution containing the electroactive species andthe system is subjected to the step-wise electrochemical deposition asdescribed above.

The resulting nanofibers grow in a radial pattern, with an orientationperpendicular to the particles' surfaces rather than the electrodesurface. The product is a film of three-dimensionally oriented nanofibernetworks that are interconnected with each other. Although the networksare interconnected, the individual nanofibers tend to retain asubstantially uniform cylindrical shape. Such films have an openstructure with nanometer-sized pores and may be especially useful foractive filtration membranes.

Unlike previous synthesis of nanofibers a template substrate is notrequired in the disclosed methods to produce oriented nanofibers. Thus,there is no need to remove a template substrate. In other words, theelectrosynthesis described herein results in an array of freestandingoriented nanofibers that do not require support along their longitudinalor elongated axis by a surrounding matrix. Moreover, since the disclosedmethod is not limited by size constraints imposed by any intricatenanostructure of a template substrate, arrays of relatively largetwo-dimensional surface areas can be produced. For example, electrodesubstrates with planar surfaces that extend over relatively largetwo-dimensional distances (e.g., from a few square centimeters up totens of square meters) may be used as the substrate for producing arraysthat are co-extensive with the electrode substrate surfaces.

The electrosynthesis disclosed herein typically occurs in the liquidphase and involves at least one electrolyte solution. Thus, thedescribed methods can be used with materials that are difficult tovolatilize since gas phase processing is not required. Moreover, themajority of useful polymers and ceramic materials are not made by gasphase reactions. Electrochemical deposition equipment also is lessexpensive compared to gas phase equipment such as chemical vapordeposition devices. The electrolyte solution includes at least onespecies having an affinity for undergoing electrochemical deposition.The electroactive species may be an inorganic or organic substance.Illustrative substances include organic polymer precursors such asmonomers, dimers or oligomers; and metallic ions such as those generatedby dissolved metallic compounds. Particularly useful substances arepolymer precursors that undergo polymerization during theelectrosynthesis to form conducting organic polymers. Conductingpolymers have the unique characteristic in that they are not alwayselectrically conductive. They are usually in a conductive state onlywhen at least partially oxidized. Reduced (i.e., neutral) conductivepolymers usually have conductivities that are several orders ofmagnitude lower than their conductivities when oxidized. Conductingpolymers typically have the desirable features of rapid response to anapplied potential (i.e., high switching speed), durability, and lowaverage power consumption under repetitive potential cycling. Possibleconducting polymers that can be synthesized into nanofibers includepolyaniline, polythiophene, polypyrrole, polyarylene, polyphenylene,poly(bisthiophenephenylene), conjugated ladder polymer, poly(arylenevinylene), poly(arylene ethynylene), organometallic derivatives thereof,and inorganic derivatives thereof. Possible metals that can besynthesized into nanofibers include gold, silver, platinum, palladium,cobalt, nickel, copper, and iridium and metal oxides such as TiO₂, SnO₂,Fe₂O₃, Fe₃O₄, and Co₂O₃.

Mixtures of electroactive species could be included in a singleelectrolyte solution. Electrolyte solutions with varying electroactivespecies also could be used in succession to produce layered films orcomposite films. The electrodeposition potential of each electroactivespecies could be different resulting in a layered film whose successivelayers range from the highest potential species at the bottom layer tothe lowest potential species at the top layer.

The amount of the electroactive species in the electrolyte solution isnot critical and may vary broadly. The minimum amount should besufficient to sustain nucleation and growth of the nanofibers. Too largeof a loading of electroactive species may lead to growth of randomlyoriented fibers.

The electrolyte solution also may include a secondary electrolyte toincrease the electrical conductivity of the solution. The amount ofsecondary electrolyte may vary, but one particular concentration rangeis about 0.1 M to about 2 M. Illustrative secondary electrolytes includeperchloric acid, sulfuric acid, hydrochloric acid, KCl, NaCl, andNaClO4.

The electrolyte solution typically is a liquid. The carrier liquid forthe electrolyte solution may be water, a polar organic liquid solventsuch as acetonitrile, or a mixture thereof Aqueous electrolyte solutionsgenerally are environmentally preferred over organic electrolytesolutions.

The electrochemical deposition can be performed as a batch process or acontinuous process. In the case of a continuous process a fresh streamof the electroactive species can be introduced at least intermittentlyto replenish the depleted electrochemical bath. Alternatively, theelectrolyte solution could continuously flow over the electrode surface.

The nanofibers in the arrays usually have substantially uniform shapesand an average diameter of less than about 1 micron. For example, thenanofibers may have a diameter of about 10 to about 200 nm, moreparticularly about 40 to about 100 nm. The length of nanofibers may varydepending upon the desired resulting film thickness. For example, thenanofibers may have an average length ranging from about 500 to about10,000 nm, more particularly about 800 to about 5000 nm.

The nanofiber arrays may form a continuous or semi-continuous filmacross the electrode substrate surface. The film thickness typicallycorresponds to the length of the nanofibers. The nanofiber films maydefine a three-dimensional nanoporous structure. The nanofiber arrays orfilms may be removed from the electrode substrate surface by anysuitable technique. For example, the films could be mechanically removedby peeling. Alternatively, the film could be removed by dissolving thesubstrate. As an illustration, the substrate could be silicon or silicondioxide that can be dissolved by an acid such as HF or a base such assodium hydroxide.

The disclosed nanofiber arrays have a number of useful properties. Forexample, nanofiber arrays made from conducting polymers are redox activematerials. In general, redox active materials are materials that cangenerate an electrical signal in response to a change in physical andelectrochemical properties caused by oxidizing and/or reducing thematerial. For example, the electrical conductivity of a redox activematerial can be reversibly altered by applying an oxidizing or reducingpotential to the electrode made from /or coated with the redox activematerial. Films of the oriented nanofibers exhibit considerably higherredox currents compared to films of randomly oriented fibers orpolymers. The improvement in the redox current reflects the orientednanofiber arrays' greater effective electrochemical active surface areasthat are accessible to the electrolytes.

The oriented nanofiber structures disclosed herein can have a multitudeof applications. For example, the conducting polymer nanofiberstructures are electroactive materials that can be employed in chemicaland biological agent sensing and diagnostic devices; energy conversionand storage devices (e.g., photovoltaic cells, batteries, capacitors,and hydrogen storage); catalysts; multifunctional photonic band gapmaterials; molecular circuit elements; permeation membranes;semiconducting devices; and light emitting diodes. The high surface areaand high porosity associated with the open nanostructures anddirectionally controllable structure orientation can offer high capacityand efficiency for energy conversion and storage, and reactioncatalysis.

One illustration of an application for the oriented nanofiber structuresinvolves synthesizing a nanocomposite structure (e.g., a film) thatincludes both the nanofiber material and at least one additionalsubstance. Electrodeposition, electroless metal deposition, adsorption,coating of an emulsion or dispersion of the particles, or othertechniques can be used to introduce the additional substance into thenanofiber array structure. For example, particles of a second substancemay be introduced into nanometer-sized pores in the nanofiber film.

In certain embodiments, particles of the additional substance are loadedor dispersed into the nanoporous structure by electrochemicaldeposition. The electrochemical deposition of the particles can beaccomplished by cycling the potential in a certain range or bypotentiostatic or galvanostatic deposition techniques. Such a methodoffers the ability to precisely control the amount of particles loadedinto the nanoporous structure. The nanoporous structures have extremelyhigh surface areas and provide superior support for a substantiallyuniform dispersion of particles into a three-dimensional network. Thehigh surface area of the nanoporous film structures results in anincreased loading capacity for the particles.

Inorganic substances that enhance the electrical properties of aconducting polymer nanofiber film are one class of additional substancesthat can make distinctive composites or matrices. The inorganicsubstance could be a metallic complex, a metal, or a metal oxide. Theexceptionally high loading capacity of the nanoporous film structuresleads to high capacity in catalysts, redox capacitors, and batteries,electrochromic systems, and increased sensitivity in chemical andbiological sensors. For example, nanoporous conductive polymer films ornanoporous films loaded with a catalyst (e.g., Pt, Pd, Ru, or TiO₂) canbe utilized as electrode materials for energy conversion devices such asfuel cells, photovoltaic cells, and hydrogen storage devices. Nanoporousconductive polymer films or nanoporous films loaded with a redox activematerial can be used as electrode materials for energy storage devicessuch as batteries and capacitors. Nanoporous conductive polymer filmscan be used as anion exchange membranes for selective separation andpurification due to the reversibility of doping/undoping of anion.

A particularly useful material that can be introduced into the nanofiberfilms is a mixed valence compound such as a metal hexacyanometallate.Transition metal hexacyanometallates may be represented by the formulaM¹M²(III)[M³(II)(CN)₆] or M²(III)₄[M³(II)(CN)₆]₃ wherein M²(III) isFe(III), Ru(III), Os(III), Co(III); Cr(III); M³(II) is Fe(II), Ru(II),Os(II), Co(II); and M¹ is an alkali metal cation. Additional usefulmixed valence compounds include molybdenum, Cu(II), Ni(II), and Mn(II)ferricyanide. An especially useful metal hexacyanometallate is iron(III) hexacyanoferrate (“FeHCF”). FeHCF is believed to exist in thesolid state in two forms: insoluble Fe₄[Fe(CN)₆]₃ and solubleKFe[Fe(CN)₆].

For example, composite films containing metal hexacyanometallates can beused in ion-sensitive membranes for determination of alkaline cations;in sensors for methanol, water vapor, and dichloroethane; and foroptical measurements. The metal hexacyanometallates-containingstructures could be used in electrochromic systems since metalhexacyanometallates (particularly iron (III) hexacyanoferrate) canquickly and reversibly change color upon application of an electricpotential. In particular, the metal hexacyanometallates are redox activematerials that are very intensely colored in one redox state, but not inanother redox state.

Electrochemical deposition of the metal hexacyanometallate can beaccomplished by contacting the nanoporous film coated on an electrodesubstrate with a solution that includes precursors of the metalhexacyanometallate. The solution typically includes a metal(III) ion, ahexacyanometallate ion and a supporting electrolyte. The amount ofmetal(III) ion and hexacyanometallate ion in the solution depends uponthe desired particle loading. For example, the solution may includeabout 0.001 to about 0.05 M metal(III) ions and about 0.001 to about0.05 M hexacyanometallate ions.

Applying a voltage or cycling a voltage range applied to the electrodesubstrate causes the in situ formation and deposition of the metalhexacyanometallate. The specific voltage applied depends upon a numberof factors including the type of materials and desired particle loading.The voltage typically is cycled between a high voltage of about 0.5 toabout 1.2 V, particularly about 0.6 to about 0.8 V, and a low voltage ofabout −0.1 to about −0.6 V, particularly about −0.2 to about −0.5 V. Thevoltage may be cycled at a rate of about 1 to about 200 mV/sec,particularly about 10 to about 100 mV/sec. The voltage cycling isapplied for a sufficient period of time to achieve the desired loadingof the metal hexacyanometallate particles. Such time periods varywidely, but may generally be about 5 to about 30 minutes.

One or more compounds that generate metal(III) ions when dissolved insolution may be used to make the electrolysis solution. Any desiredcompound may generally be used to form the metal(III) ion-containingsolution as long as it can dissociate metal(III) ion in the solution.Illustrative compounds include metal(III) salts of inorganic acids suchas metal(III) chloride, metal(III) sulfate, metal(III) perchlorate,metal(III) nitrate, metal(III) phosphate, or metal(III) pyrophosphate;metal(III) salts of organic acids such as metal(III) oxalate, metal(III)acetate, metal(III) citrate, metal(III) lactate, or metal(III) tartrate;and metal(III) ammonium double salts such as metal(III) ammoniumsulfate, metal(III) ammonium oxalate, or metal(III) ammonium citrate.

Any desired compound may generally be used to form thehexacyanometallate ion-containing solution as long as it can providehexacyanometallate ion in the solution. Illustrative compounds includepotassium hexacyanometallate, sodium hexacyanometallate, lithiumhexacyanometallate, rubidium hexacyanometallate, and ammoniumhexacyanometallate.

The supporting electrolyte may be added for ensuring consistentelectrolysis by adjusting the acidity of the solution, increasing theconductivity of the solution and improving the chemical stability ofsolutes. Exemplary supporting electrolytes include at least one compoundselected from inorganic and organic acids such as hydrochloric acid,sulfuric acid, perchloric acid, nitric acid, phosphoric acid,pyrophosphoric acid, hexafluorophosphoric acid, boric acid,tetrafluorophosphoric acid, carbonic acid, oxalic acid, acetic acid,citric acid, lactic acid, tartaric acid, or phthalic acid, and salts ofthe foregoing inorganic and organic acids with lithium, sodium,potassium, rubidium, calcium, magnesium, ammonium, ortetraalkylammonium. Buffers and pH adjusters may also be added to theelectrolysis solution.

The carrier fluid which may be used in preparing the metal(III)ion-containing and hexacyanoferrate(III) ion-containing solutiontypically is a solvent such as water or a water/acetonitrile mixture,although polar organic solvents such as acetonitrile, tetrahydrofuran,or N,N-dimethylformamide may also be used. Any solvent may be used aslong as the iron(III) ion, hexacyanoferrate(III) ion and optionalsupporting electrolyte are stable in the solvent. The solution typicallyis made by simply mixing together the various ingredients.

The conducting polymer/inorganic composite films have applications in avariety of fields such as sensors, catalysts, redox capacitors, andsecondary batteries. One illustration of an application is in theconstruction of a biological or chemical sensor that capitalizes on thecatalytic properties of the conducting polymer/inorganic compositefilms.

As described above, amperometric detection of peroxides such as hydrogenperoxide or organic peroxides is becoming increasingly useful.Additional analytes or target compounds of interest for sensors includeother electroactive organic compounds such as phenols. Illustrativeorganic peroxides include organic (hydro)peroxides such as benzoylperoxide, cumene hydroperoxide, t-butyl hydroperoxide, 2-butanoneperoxide, and fatty acid hydroperoxides (e.g., linolenic acidhydroperoxide and lauroyl peroxide).

The conducting polymer/inorganic composite films produced by the methodsdisclosed herein are especially suitable for constructing sensors forsuch detection. The films can be deposited on an electrode substrate. Anelectrolyte solution that includes the analyte of interest is broughtinto contact with the modified electrode substrate. An electricalpotential is applied to the electrode and the analyte undergoesoxidation or reduction that is detected by the electrode. One feature ofsuch sensors is the ability to accurately detect hydrogen peroxide at alow potential such as, for example, about 0.2 to about −0.2 V. Such lowpotential substantially eliminates the interference from othercomponents in the analyte sample. Another feature is the improvedstability of the metal hexacyanometallate coating due to the supportprovided by the nanoporous matrix. A further characteristic of thesensors is a greater density of sensing sites (i.e., thethree-dimensionally distributed metal hexacyanometallate particles) dueto the three-dimensional access to the sensing sites by the analyte.Consequently, a greater total sensing surface area is available in agiven two-dimensional surface area leading to sufficient sensitivity forminiaturization of the sensor. The analyte detection could bequantitative as well as qualitative.

One possible system for amperometric detection is illustrated by theflow injection analysis system shown in FIG. 7. The system includes avessel 1 containing a carrier fluid mixture (e.g., water with bufferingagents). The vessel would be fluidly coupled to a pump 2 that wouldtransport the carrier fluid mixture through a sample injector 3. Thesample injector 3 would include an inlet for receiving samples so thatthey could be mixed with the carrier fluid and an outlet for any wastestreams. The resulting sample/carrier solution then is introduced into amicroelectrochemical cell 4. A computer 5 is coupled to themicroelectrochemical cell 4 to control the microelectrochemical cell 4.

An example of a microelectrochemical cell 4 is depicted in FIG. 8 and inLin et al., “Integrated Microfluidics/Electrochemical Sensor System forMonitoring of Environmental Exposures to Lead and Chlorophenols”Biomedical Microdevices: 3(4):331-338, 2001. The microelectrochemicalcell 4 defines an inlet 10 for receiving the sample/carrier solution, aflow channel 11 that directs the sample/carrier solution over a workingelectrode 12, a reference electrode 13, and an auxiliary electrode 14,and an outlet 15 for discharging the sample/carrier solution. Theworking electrode 12 (i.e., the sensor) is a gold (or Pt, glassy carbon,graphite) disc embedded in a polymer plate (e.g., polyetheretherketone(“PEEK”)). A transition metal hexacyanometallate/nanoporous conductivepolymer film composite modifies the surface of the working electrode 12.The modified working electrode 12 surface is an electron transfermediator for catalytic reduction or oxidation of target analytes on theelectrode surface. The reduction/oxidation current (usually at nA to μArange) is converted into voltage and amplified by a signal amplifier.The reduction/oxidation current is proportional to the analyteconcentration in solution.

A biosensor also could be constructed from the modified electrodesdisclosed herein by immobilizing an enzyme or other biological reagenton the electrode surface by techniques known in the art as described,for example, in Karyakin et al., “Prussian Blue-Based First-GenerationBiosensor, A Sensitive Amperometric Electrode for Glucose”, Anal. Chem.,67, 2419-2423, 1995.

Another illustration of an application of the conductingpolymer/inorganic composite films is in the separation of cesium ionsfrom a nuclear waste stream or groundwater contaminated with cesium.Transition metal hexacyanometallates are very selective for cesiumsorption since the cesium may be substituted for sodium or potassiumions in the transition metal hexacyanometallate (see Rassat et al.“Development of an electrically switched ion exchange process forselective ion separations”, Sep. Purif Technol., 15, 207-222, 1999;Yuehe Lin et al., “Selective Sorption of Cesium Using Self-AssembledMonolayers on Mesoporous Supports (SAMMS)”, Environmental Science &Technology, 35, 3962-3966, 2001). As mentioned above, the nanoporousmatrix provides a greater loading density of transition metalhexacyanometallates leading to a greater separation capacity.

The specific examples described below are for illustrative purposes andshould not be considered as limiting the scope of the appended claims.

EXAMPLE 1 Synthesis of Nanofibers

A Pt plate about 1 inch by 1 inch in size was washed thoroughly withethanol and dried in air. The Pt plate was further rinsed in a 1 wt. %sodium dodecyl sulfate (SDS) solution and dried in air to improve thewetting behavior with water. Electrochemical deposition of polyaniline(PANI) was performed by immersing the Pt plate into an aqueous solutioncontaining 0.5 M aniline monomer and 1.0 M perchloric acid (HClO₄). Theeffective area of the immersed Pt plate was 4.5 cm². Polyaniline wasgrown from the surface of the Pt plate by redox polymerization ofaniline. The electrochemical experiments were performed on an EG & GPrinceton Applied Research model 273 potentiostat/galvanostat controlledby a personal computer via EG & G Princeton Applied Research Model 270electrochemical software. The experiments for PANI film depositions weremade in an H-shape two-compartment cell, with another platinum plateused as the counter electrode. A saturated calomel electrode (“SCE”) islocated in the cathode part of the H-shape cell. The electrochemicaldeposition of PANI films was performed by a programmed constant-currentmethod, designed to control the nucleation and growth rate. A step-wiseprocedure with multi-step changes of current density was employed tocontinually deposit polyaniline on the surface of the Pt plate. Thefollowing procedure was used to prepare the samples: 0.08 mA/cm² for 0.5hours, followed by 0.04 mA/cm² for 3 hours, which was followed byanother 3 hours at 0.02 mA/cm². At the end of each step, the sample wasrinsed with deionized water to remove electrolyte solution and monomerattached to the surface of the Pt plate. Besides Pt, other substrates,including Ti, Au, and Si/Au, were also investigated and found to yieldsimilar results.

FIGS. 1A-1D show the oriented nanofibers or nanowires produced by theabove-described experiment. When viewed from an angle perpendicular tothe surface at a lower magnification (FIG. 1A), the film appears tocontain uniform white spots all across the surface. At a highermagnification (FIG. 1B), it is revealed that the white spots areactually the tips of uniform nanofibers or nanowires, mostly orientedsubstantially perpendicular with respect to the substrate. The diametersof the tips range from 50 nm to 70 nm. Some thin filament structures,about 20 nm in diameter, can also be observed at the base of theoriented nanofibers or nanowires. When the sample is tilted, themorphology and the orientation of the nanofibers or nanowires areclearly revealed (FIGS. 1C and 1D). The oriented nanofibers or nanowiresare fairly uniform in length and diameter, but the diameter is slightlysmaller at the tip position than at the base position. Judging from thetilt angle (about 40°), the nanofibers or nanowires are about 1 μm inlength.

A sample after only the first deposition step (i.e., 0.08 mA/cm²) isshown in FIG. 2A. At this stage, the polymer was deposited on thesurface as small particles about 50 nm in diameter. No extendednanofibers were formed yet.

Controlled comparative experiments were also conducted to study thedeposited substance morphology without the step-wise control of thecurrent density. In these comparative examples, the polyaniline filmswere prepared by electrochemical deposition at 0.08 mA/cm2 for anextended period of time (i.e., for over 5 hours). FIG. 2B shows theresult of this comparative sample. A much higher density ofnanoparticles can be observed, but there are no oriented nanofibers.Moreover, thick worm-like polymers fibers in the excess of a few hundrednm began to form at many locations.

In order to vary the orientation of the polymer nanofibers, a monolayerof closely packed colloidal silica (0.5 μm in diameter) was deposited onthe Pt substrate prior deposition of the aniline. Specifically, afterthe Pt plate was rinsed with the SDS solution, a droplet of a colloidalsilica solution containing about 0.4 wt. % silica particles was placedon the Pt substrate. The SDS treatment improved the wetting behavior ofthe silica colloidal solution and allowed the droplet to spread over theentire surface of the Pt plate. The excess solution was removed from thesubstrate by positioning the Pt plate in a vertical position thatallowed the excess water to flow off. After drying, the Pt plate withthe silica particles was heated in an oven at 110° C. for 0.5 hour.Electrochemical deposition of polyaniline was conducted using the sameprocedure as described above.

FIG. 3A shows the results in a substrate area where the silica particleswere not closely packed. The surface roughness induced by the presenceof the silica particles and the radial growth of the polyanilinenanofibers is clearly visible. FIG. 3C depicts the morphology of thenanofibers across the edge of the silica monolayer. Close to the frontwhere there are no silica particles, the polymers are orientedvertically. But on top of the silica particles, the nanofiberorientation is disrupted and randomly connected. In a substrate areaoccupied by a monolayer of densely packed silica spheres, the radialgrowth of polymer nanowires overlaps and forms three-dimensionallyinterconnected polymer networks (see FIGS. 3B and 3D).

The same instrument and samples were used for voltammetry experimentsusing SCE as the reference electrode. Cyclic voltammograms as shown inFIG. 4 were collected in 1 M HClO₄ and at a sweep rate of 50 mV/s. Curve(a) represents the unidirectionally oriented nanofibers on the Ptsubstrate, (b) represents the randomly oriented nanofibers on the silicaspheres, and (c) represents a conventional polyaniline film depositedwithout the step-wise current process (i.e., with a constant current).All the samples demonstrated electrochemical activity characterized bythe typical reduction and oxidation peaks. However, the unidirectionallyoriented nanofibers have considerably higher redox currents compared tothose of randomly oriented fibers, which in turn have a higher redoxcurrent than the samples prepared with a constant current. Thedifference in the redox currents reflects the effective electrochemicalactive surface areas that are accessible to the electrolytes. Based onthese results, it is apparent that the substantially parallel orientednanowires have the highest effective surface area, which is desirablefor high efficiency and sensitivity devices.

PANI nanofibers were also deposited on silicon wafers following theprocedure described below. The silicon wafers were supersonicallycleaned for 15 minutes in 2-isopropanol to remove any trace organicresidue. The pre-cleaned silicon wafers were immersed in 0.1 M KOH for 2minutes and 0.1 M HNO₃ for 10 minutes. The wafers were subsequentlywashed with excess water. The wafers then were dried under flowing N₂for a minimum of 2 hours prior to coating with gold via sputtering.After the gold sputtering, the surface of the wafer was coated with a 1wt. % solution of sodium dodecyl sulfate. The wafer was dried in an ovenat 110° C. for 0.5 hour. The PANI electrochemical deposition synthesisdescribed above was then performed on the prepared silicon wafersresulting in the similar formation of oriented PANI nanofibers.

EXAMPLE 2 Synthesis of Composite Films

A polyaniline nanoporous film was synthesized according to Example 1,except that the second and third constant current density steps weremaintained for four hours rather than three. In addition, the workingelectrode was a glassy carbon disk with a surface area of 0.14 cm²(commercially available from Bioanalytical Systems) rather than a Ptsubstrate.

FeHCF was electrodeposited on the polyaniline-modified electrode byimmersing the electrode in a 0.1 M KCl aqueous solution containing a0.01 M equimolar mixture of FeCl₃ and K₃Fe(CN)₆. The electrode potentialwas cycled between −0.20 and +0.80 V at 50 mV/s in the mixed solution of0.1 M KCl, 0.01 M FeCl₃ and 0.01 M K₃Fe(CN)₆.

The surfaces of the polyaniline/FeHCF-modified electrode wereinvestigated with SEM. The SEM micrographs indicated that FeHCF wasdeposited into the nanoporous polyaniline matrix and at least partiallyfilled the nanopores. The surfaces of the polyaniline/FeHCF-modifiedelectrode were also investigated with x-ray photoelectron spectrometry(“XPS”). The XPS spectra clearly showed Fe peaks confirming thedeposition of FeHCF into the polyaniline matrix.

The synthesis and testing of the polyaniline/FeHCF films were performedusing a CH Instruments model CHI 660 potentiostat controlled by apersonal computer and the electrochemical software. The experiments weremade in a one-compartment cell containing three electrodes. Thepotential of the working electrode (i.e., the polyaniline-coatedelectrode) was always measured against the Ag/AgCl reference electrode.The counter electrode was a platinum wire.

EXAMPLE 3 Sensor

A polyaniline/FeHCF-modified glassy carbon electrode made as describedabove in Example 2 was used to detect the presence of hydrogen peroxideby sensing catalytic reduction of hydrogen peroxide. The amperometricdetection was conducted using the flow injection analysis system shownin FIG. 7. The particular system included a peristaltic pump, a Rheodane7125 injector with a 50-μL sample loop, an interconnectingpolytetrafluorethylene tubing, and a thin-layer electrochemical flowcell. Flow injection/amperometric measurements were conducted with a CHInstruments model CHI 824 electrochemical detector. The workingelectrode is the glassy carbon electrode embedded in a PEEK plate. Aconstant potential of 0.1 V was applied to the electrode. The flow rateof the carrier solution (0.1 M KCl, 0.05 M acetate buffer (pH 6.0) was0.5 mL/min. A 50-uL sample containing hydrogen peroxide was injectedfrom the injection valve into the carrier solution. Hydrogen peroxidewas reduced when sample flowed through the electrode surface. Thereducing current (signal) was used to measure the concentration ofhydrogen peroxide. The hydrogen peroxide concentration varied asindicated in FIG. 5.

Curve B of FIG. 5 shows the currents generated by the catalyticreduction of hydrogen peroxide by the working electrode. Curve A of FIG.5 shows the currents generated by the catalytic reduction of hydrogenperoxide by an electrode that was modified by forming a polyaniline filmon the electrode surface that did not include FeHCF. These resultsclearly indicate that FeHCF deposited into the nanoporous PANI film actsas an electron transfer mediator between the electrode and the hydrogenperoxide. Without deposition of FeHCF, the PANI-modified electrode has avery low response to hydrogen peroxide at a low potential. Withdeposition of FeHCF, the electrode has a high response to hydrogenperoxide at a low potential. Application of such a low electrodepotential (0.10 V) can eliminate the interference from anodic dischargeof many components often present in analyte solutions.

FIG. 6 shows the polyaniline/FeHCF-modified glassy carbon electroderesponse to six injections of 50 ppm hydrogen peroxide solution. Theoperating conditions were the same as described above in connection withFIG. 5. Well-defined peaks are observed at a low potential (0.10 V)indicating the reproducibility of the sensor.

Having illustrated and described the principles of the disclosed methodsand substrates with reference to several embodiments, it should beapparent that these methods and substrates may be modified inarrangement and detail without departing from such principles.

1-39. (canceled)
 40. An array of oriented nanofibers produced by amethod comprising: forming a solution that includes at least oneelectroactive species; contacting the solution and an electrodesubstrate; and applying a current to the electrode substrate thatincludes at least a first step of applying a first substantiallyconstant current density for a first time period and a second step ofapplying a second substantially constant current density for a secondtime period, wherein the first substantially constant current density isgreater than the second substantially constant current density; andwherein the first time period and second time period are of sufficientduration to electrically deposit on the electrode substrate an array oforiented nanofibers produced from the electroactive species.
 41. Asubstrate defining at least one surface having deposited thereon anarray of freestanding oriented organic polymer nanofibers, wherein thearray was produced by liquid phase processing and without a template.42. The substrate of claim 41, wherein the organic polymer nanofiberscomprise a conducting polymer.
 43. The substrate of claim 42, whereinthe conducting polymer comprises a polyaniline, polythiophene,polypyrrole, polyarylene, polyphenylene, poly(bisthiophenephenylene),conjugated ladder polymer, poly(arylene vinylene), poly(aryleneethynylene), organometallic derivative thereof, or inorganic derivativethereof.
 44. The substrate of claim 41, wherein the oriented nanofibersare substantially unidirectionally oriented.
 45. The substrate of claim41, wherein the oriented nanofibers are three-dimensionally oriented.46. The substrate of claim 44, wherein the unidirectionally orientedfibers are substantially perpendicular to a plane formed by thesubstrate surface.
 47. The substrate of claim 41, wherein the nanofibershave an average diameter of less than about 1 micron and an averagelength of about 500 to about 10,000 nm.
 48. A film comprising a networkof three-dimensionally oriented conducting polymer nanofibers, whereinthe individual nanofibers have a substantially uniform cylindrical shapeand there is substantially no branching of the individual nanofibers.49. The film of claim 48, wherein the nanofiber network definesnanometer-sized voids and further comprises a second substance at leastpartially received within the nanometer-sized voids of the nanofibernetwork.
 50. The film of claim 49, wherein the second substancecomprises a metallic material.
 51. The film of claim 50, wherein thesecond substance comprises a metal hexacyanometallate.
 52. The film ofclaim 51, wherein the metal hexacyanometallate comprisesM¹M²(III)[M³(II)(CN)₆] or M²(III)₄[M³(II)(CN)₆]₃ wherein M²(III) isFe(III), Ru(III), Os(III), or Co(III); M³(II) is Fe(II), Ru(II), Os(II),or Co(II); and M¹ is an alkali metal cation.
 53. The film of claim 52,wherein the metal hexacyanometallate comprises iron (III)hexacyanoferrate. 54-61. (canceled)